The Solar System’s Most Spectacular Geology Revealed by 50 Years of Robotic Exploration

Before 1962, most of the planets in our solar system appeared as hardly more than blurry dots in some astronomer’s telescope.

The most that scientists knew about Mercury, Venus, or Jupiter was their size, surface temperature, and atmospheric composition. But on Dec. 14, 1962, the Mariner 2 spacecraft flew by Venus. For the first time, researchers had detailed and up-close information about another world, helping spawn new scientific fields such as astrogeology and modern planetary science. The planets in our solar system changed from distant points to fully fledged worlds, with distinctive and amazing features.

Image: A roll of data from Venus from the 1962 Mariner 2 mission. NASA

“As Carl Sagan used to say, only one generation of humankind can be the first explorers of the solar system, and we are that generation,” USGS astrogeologist Michael Carr wrote in a recent issue of the American Geophysical Union's weekly magazine Eos, about the last 50 years of solar system exploration.

Scientists have gained an incredible amount of information and understanding in the half-century since Mariner 2’s flight. Here we take a look at some of the highlights from the wealth of information that has come to us from five decades of robotic spaceflight. In this gallery, we can see how our knowledge has grown in leaps and bounds during the last 50 years by looking at some important firsts in planetary exploration as well as some of the biggest, strangest, and most impressive geologic structures in the solar system.

Above:

What Makes Up the Moon

In 1992, the Jupiter-bound Galileo spacecraft made a pass by our planet’s closest companion, the moon. This mosaic of 53 images shows the different composition of rocks on the moon's surface. Blue and orange colors represent lava flows, bright pink areas are highlands, and light blue colors indicate recent impact material with the youngest craters showing blue rays extending away from them.

Early Mercury

This mosaic was made from Mariner 10 data, taken in 1974 and 1975 when the spacecraft flew by Mercury, giving scientists their first close-up glimpse of the innermost planet. The geology looks similar to that of the moon, with smooth lava plains and impact craters dominating the terrain. More than three decades Later, the MESSENGER mission would reveal even more about the surprisingly interesting little planet.

Beagle Escarpment

When the MESSENGER spacecraft flew past Mercury for the first time in 2008, it spotted a brand new feature: Beagle Rupes. The longest escarpment on the planet, Beagle Rupes runs for 600 km (372 miles) over Mercury’s surface. Mercury's escarpments are thought to be caused by thrust faults, where part of the crust is thrust on top of another part, created when the planet cooled and contracted.

Early Venus Picture

This ultraviolet view of Venus’ clouds comes from the 1974 Mariner 10 flyby. It shows the surface of the planet perpetually blanketed by clouds, which contribute to its hellish surface temperatures in excess of 460 degrees Celsius, hot enough to melt lead. It would not be until the Magellan spacecraft reached Venus in 1990 that we would get a detailed look what was beneath those clouds.

Venus revealed

This image was made from radar data taken in 1991 by the Magellan spacecraft, which eventually mapped 98 percent of the planet's surface. The bright region at the center of the image is a rugged highland area known as Aphrodite Terra.

Venus has evidence of a long history of volcanic activity and has shield and composite volcanoes similar to ones found on Earth. The planet has fewer impact craters than the moon or Mercury, probably because the dense atmosphere burns up smaller asteroids before they reach the surface. Venus also has some strange circular and oval features known as coronae, which are thought to be formed by hot, molten material upwelling from below, similar to hotspots on Earth that form volcanic islands in the ocean crust. On Venus, the rising, molten plumes push the crust up into a dome and then cool off causing the domes to collapse into depressions.

First Mars Image

One of the earliest images from Mars is this strange picture from the Mariner 4 mission in 1964. Too anxious to wait for the official processed shot from the spacecraft, engineers at JPL simply converted the image data into integers and then hand colored it in like a paint-by-numbers drawing.

Mariner 4 transmitted the first close-up pictures of another planet and revealed Mars to be a dry, desert-like world.

Olympus Mons

The biggest known volcano in the solar system is Olympus Mons on Mars, discovered by Mariner 9 in 1971. The mountain towers nearly 22 km (14 miles) above the surrounding plain, more than three times Mount Everest's elevation above sea level. It is more than 600 km across and occupies an area similar in size to Arizona. The cliff around the outer edge of the volcano is 6 km (3.7 miles) high. Olympus Mons is a shield volcano, similar to the much smaller Hawaiian Islands on Earth.

The image above is a mosaic from images taken by the Viking 1 orbiter in 1978. Below is a computer-generated image of Olympus Mons based on thousands of laser-altimeter elevation measurements made by the Mars Global Surveyor spacecraft.

Volcanic Io

When NASA’s Voyager 1 mission zipped by the Jupiter system in 1979, it discovered that far from the sun were worlds of incredibly active worlds. In particular, it shot spectacular pictures of the moon Io, which is covered in volcanoes and is now known to be one of the most geologically active places in the solar system. This image shows a plume from a volcano on Io’s surface.

Europa’s Icy Surface

Scientists suspect the icy crust of Jupiter’s moon Europa hides a deep liquid ocean. The ice and ocean together may be as deep as 100 km, covering a rocky surface below. The rocky part of the moon is likely similar in composition to Earth, which is made mostly of silicate rock.

The smoothness and low density of impact craters suggests the surface of the ice is young and probably being actively resurfaced. The dark linear stripes on Europa, clearly visible in the images above from the Galileo spacecraft in 1996, could be caused by cracking due to tidal motion of the liquid beneath. The cracks may allow water through to the surface, similar to magma erupting onto Earth's sea floor along the mid-ocean ridges.

The image below is also based on data from Galileo, collected in 1995 and 1998.

Iapetus’ Ridge

Saturn’s moon Iapetus has one of the most spectacularly strange geologic features in the solar system, a ridge 20 km high that runs for 1,300 km along the moon’s equator.

The strange ridge couldn't have formed the way mountains on Earth are built, by tectonic plates colliding, because Iapetus would have other mountains north and south of the equator. Theories about how the weird ridge was formed include a moonlet orbiting around the moon that fell toward the moon, was shredded by the tidal forces of the larger moon, broke into a ring of even smaller moonlets which then crashed into the surface. Another theory holds that a giant impact with another object slowed the moon's rotation which deformed it into a walnut shaped body with a ridge.

Miranda’s Chevrons

Our only close-ups of Uranus’ moon Miranda come from Voyager 2's flyby in 1986. The interesting surface of the moon may be made of ice and is a jumbled, faulted assemblage of older, heavily cratered terrain, interspersed with young land covered in scarps and ridges and features that may have been formed by upwelling of warmer water. One of Miranda’s distinctive “chevron” features, a v-shaped landscape, can be seen in the image.

Triton’s Cryovolcanoes

Scientists got their first glimpse of Neptune’s moon Triton with the Voyager 2 spacecraft in 1989. The relative lack of impact craters indicates the moon’s surface is young, which means it is geologically active. The crust is likely made of different types of ice made of nitrogen water and carbon dioxide and is resurfaced by cryovolcanism. Much of the moon's mass may be water ice with the rest being rocky. Triton's similarity to Pluto suggests it may have been captured by Neptune from the Kuiper belt.

South-Pole Aiken Basin

One of largest impact craters in the solar system is the lunar South Pole Aiken Basin. Located on the far side of the moon (which never faces the Earth) it was only possible to discover this feature with robotic probes. Since the mid-1960s, scientists have recognized its enormous dimensions: roughly 2,500 km across, while the moon itself is only 3,500 km in diameter to begin with.

The image above comes from the Lunar Reconnaissance Orbiter, one of NASA’s most recent missions. It contrasts well with the first image of the moon taken by a U.S. spacecraft — the Ranger 7 mission of 1964 – which shows several craters (below). The large crater in the center right is called Alphonsus, flanked above by Ptolemaeus and below by Arzachel.

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